to be in different phases of vibration. If one of the particles lie
upon the crest, and the other on the furrow of the wave, then, as one
is about to rise and the other about to fall, they are said to be in
_opposite_ phases of vibration.

There is still another point to be cleared up--and it is one of the
utmost importance as regards our present subject. Let O (fig. 17) be a
spot in still water which, when disturbed, produces a series of
circular waves: the disturbance necessary to produce these waves is
simply an oscillation up and down of the water at O. Let _m_ _n_ be
the position of the ridge of one of the waves at any moment, and _m'_
_n'_ its position a second or two afterwards. Now every particle of
water, as the wave passes it, oscillates, as we have learned, up and
down. If, then, this oscillation be a sufficient origin of
wave-motion, each distinct particle of the wave _m_ _n_ ought to give
birth, to a series of circular waves. This is the important point up
to which I wish to lead you. Every particle of the wave _m_ _n_ _does_
act in this way. Taking each particle as a centre, and surrounding it
by a circular wave with a radius equal to the distance between _m_ _n_
and _m'_ _n'_, the coalescence of all these little waves would build
up the large ridge _m'_ _n'_ exactly as we find it built up in nature.
Here, in fact, we resolve the wave-motion into its elements, and
having succeeded in doing this we shall have no great difficulty in
applying our knowledge to optical phenomena.

[Illustration: Fig. 17.]

Now let us return to our slit, and, for the sake of simplicity, we
will first consider the case of monochromatic light. Conceive a series
of waves of ether advancing from the first slit towards the second,